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'Compensatory lengthening' in phonology and historical linguistics is the lengthening of a vowel sound that happens upon the loss of a following consonant, usually in the syllable coda. An example from the history of English is the lengthening of vowels that happened when the voiceless palatal fricative and its allophone were lost. For example, in Chaucer's time the word ''night'' was pronounced ; later the was lost and the was lengthened to by compensatory lengthening. (Later the became by the Great Vowel Shift.)
Both the Germanic spirant law and the Ingvaeonic nasal spirant law show vowel lengthening compensating for the loss of a nasal.
Non-rhotic forms of English have a lengthened vowel before a silent post-vocalic r: in Scottish English, ''girl'' has a short /i/ followed by a light alveolar /r/, as presumably it did in Middle English; in Southern British English, the /r/ has dropped out of the spoken form and the vowel has become a "long schwa".